Noticed the same thing with a mission I did the other day. If you can get me the following info then I can take a further look at why this happens.

Total force size for all sides (e.g. west => 400, east => 400)
Force weightings for all place modules
Total number of objectives
Number of objectives occupied by each side at start (e.g. west => 8, east => 16)
Reinforcement settings in the commander modules
Anything else you think might be helpful

Hmm with those numbers I don't think it's related to what I was thinking. Can you try upping the squad numbers to say 20 and see if reinforcements start to work?

I'll check out the mission later today to see if there's anything obvious.

I've had the same problem with small numbers of groups. They tend to not try to hold the military obj's necessary for the reinforcements. I'd definitely try upping the force numbers and also consider adding a cust obj set to spawn a unit or two over a mil object itself.

What about about having a unit or two just hang back and hold the military HQ or obj? They'll never reinforce if they don't hold a military obj.

If it helps any, I had a similar thing happen in a mission on Takistan. The mil log side (BLUFOR) had plenty of units so I knew that wasn't the issue. But they never would replenish. They'd just deplete and deplete until the battle was too lopsided.

What I realized was that essentially, I had blacklisted both airports/military bases from BLUFOR so they never had any suitable place to replenish from. The second I added custom objective set to spawn at least 1 BLUFOR unit over one of the bases, BLUFOR finally 'held' the base and reinforcements would arrive no problem.

I'd suggest either starting both sides from large military objective (such as a base with a runway for planes) or placing a HQ set to spawn close to the module and make sure units are set to occupy them.

Reinforcements will only be allowed to come if OPCOM has a suitable military objective under their control. The easiest way would be for that objective to be their own base.

A runway isn't required, just make sure that the commander has control over a mil objective. Apparently.

From the wiki:

Dynamic - LOGCOM chooses the best insertion point from available friendly held objectives. High Priority objectives such as airfields and military bases with heli pads are preferred. If there are no suitable locations, LOGCOM will look for low priority military establishments or marine civilian locations (sea ports). These have a significantly lower logistics capacity than major military bases and resupply will take longer. Location start is the closest held objective in "reserve". The state of the objective is key, you won't get reinforcement start there if the objective is under attack etc.

Fixed - Static selection of logistics insertion point. LOGCOM will find the closest High Priority Military Objective to the location of the Logistics module on the map. This must be held by Friendly Forces. If the location is lost, Logistics will cease to function until the objective is recaptured. The objective must be under side control and in "reserve."

Tip: use Military Placement (Custom Objectives) set to very high priority (e.g. 999) to set a custom fixed insertion point.

Turn on debug for the military objective module, take a peek in game, and make sure both sides have units spawning inside some of the larger military objectives.

If you need to, add a custom objective module for each OPCOM set to spawn units inside their respective military objectives. I'd say 2 infantry units each would be enough to consider it held. So, for example after you debug, if each side has an air field, let each side spawn units in the airfield.

Doesn't sound like you're doing anything wrong... the crew exiting at height is a known issue and doesn't usually happen. Not sure why you just got arty vehicles but if you are getting any replacements then it is working.